Infrared detectors are widely used for collecting image information under conditions which do not allow regular optical observation, such as at night or through clouds, haze or dust. For example night vision detectors allow car drivers to watch beyond the reach of the car headlamps illumination field, for search and rescue operations and other applications.
Traditionally the infrared band is divided to Near IR (NIR) between 0.75 μm and 1.1 μm, Short Wavelength IR-1 (SWIR-1) optical band about 1.1 μm-1.7 μm, and Short Wavelength IR-2 (SWIR-2) optical band about 1.7 μm-2.5 μm, Mid Wave IR (MWIR) between 3 μm and 5 μm and Long wave IR (LWIR) between 8 μm and 14 μm.
SWIR detectors are used for various civilian and military applications including night vision, Silicon wafer inspection, laser beam monitoring, gas monitoring and other. Detection in the 1 μm-2.5 μm spectral range is usually implemented using detectors cooled (to cryogenic or non-cryogenic temperatures (such as photovoltaic (PV) detectors) and un-cooled detectors (such as bolometric detectors, photoconductor detectors, etc.). As known in the art, cooling the detector increases the system size, weight, cost and power consumption of the system. Therefore, cooled detectors are not a viable solution when these resources are limited. On the other hand, conventional un-cooled detectors have insufficient sensitivity and dark/background current, which are insufficient for night vision in the 1-2.5 μm spectral range.
Passive Night vision devices are usually categorized as either “reflective” (e.g.—utilizing light emitted by a natural or environmental source such as the moon, starlight, airglow or zodiacal light, urban ambient background illumination and reflected off the surface of the imaged objects and their background), or “radiative” (or “thermal”) that utilize optical radiance emitted by the imaged objects themselves and emitted by their background).
Imaging means belonging to the category of reflective devices include, for example imagers working in the visible and NIR bands, e.g., SLS (Star-Light-Scope), NVG (Night Vision Goggles), ICCD (Intensified CCD), EMCCD, EBCCD, and also imagers sensitive to visible through SWIR-1 bands (Germanium-CMOS, InGaAs PV FPA, Se-based Vidicons, “Black Silicon”).
Imaging means belonging to the category of “radiative” or “thermal” include, for example, Cryogenically cooled PV focal plane arrays (FPAs) based on Mercury cadmium telluride (MCT) or InSb material alloys, as well as uncooled FPAs based on bolometric, thermo-electric or piezoelectric effects.
These types of imaging techniques have several disadvantages, such that neither one of them is considered superior in all respects and all scenarios.
General Description
There is a need in the art for, and it would be useful to have, a novel SWIR imager that can operate at or near room temperature with long cut-off wavelengths that extended from 1.7 μm (for InP lattice matched InGaAs) to beyond 2.0 μm. It is believed that such SWIR imagers can be implemented by using strained InGaAs epitaxial growth on InP substrate, or MCT.
Passive night vision in the SWIR band requires very low dark current levels (noise), whereas the dark current increases as the cutoff wavelength increases. In order to allow for close to room temperature operation of a SWIR detector, and more significantly with extended wavelength SWIR, the dark current of the device must be limited. High dark current causes deterioration of the signal to noise ratio of the device, it increases the residual non uniformity of detector array and it reduces the dynamic range of the system due to integration capacitor flooding.
It would be advantageous to have super-lattice (MQW) structured P-i-N PD pixels that may be implemented in a 2D FPA, operating near zero voltage bias, to implement a dual-mode (reflective/refractive) night vision imaging means.
It would still be advantageous to have imaging means optics operating in such a manner that the mostly reflective information (in the SWIR-1 sub-band) or the mostly thermal information (in the sWIR-2 sub-band) would have different focal planes and thus could be taken in and out of focus by the user, thereby creating a picture which is mostly reflective or mostly thermal, as desired.
It would yet be advantageous to have a spectral band filter located in an optical path of light propagating to the detector operating in such a manner that the mostly reflective information (in the SWIR-1 sub-band) or the mostly thermal information (in the sWIR-2 sub-band) will be separated from each other, thereby creating a picture which is mostly reflective or mostly thermal, as desired.
The present invention partially eliminates disadvantages of the conventional SWIR imagers and provides a new night vision system which enables operation at or near room temperature and operating in the wavelength region up to 2.5 microns. The night vision system comprises an optical system for collecting light and focusing collected light onto a photodetector. The night vision system includes a photodetector having a cut-off wavelength of 2.5 microns under the room temperature conditions. The night vision system also includes an optical system configured for collecting light and focusing the collected light onto the photodetector, and a spectral filter located in an optical path of light propagating toward the photodetector. The spectral filter is configured and operable to selectively filter out light of wavelength shorter than a predetermined value. According to an embodiment, the predetermined value of the light wavelength shorter than which is filter out is about 1.8 microns. This feature enables the night vision system gradually shift operation from mostly reflection mode to a combined reflection and thermal mode, thereby allowing the night vision system to detect light reflected from and emitted by the object being imaged.
According to an embodiment, the photodetector is an extended SWIR sensor having a p-i-n structure.
According to an embodiment, the photodetector is a detection array comprising a doped InP substrate layer and a photodiode structure supported by the substrate InP layer. The said photodiode structure comprises an n-type layer, a p-type layer, and an absorption layer being sensitive to light, and located between the n and the p layers.
According to an embodiment, the n-type layer is Si-doped InGaAs buffer layer deposited on the InP substrate layer; wherein the n-type layer is undoped InGaAs cap layer; and wherein the absorption layer is undoped InGaAs.
According to one embodiment, the absorption layers is a Strained Super Lattice (SLS) layer including 250 pair InGaAs(5 nm)/GaAsSb(5 nm) multi quantum well structures.
According to another embodiment, the absorption layers is a Strained Super Lattice (SLS) layer including three superlattice layers SL1, SL2 and SL3 grown on the InP substrate layer. The SL1 superlattice layer includes: 500 periods of In0.53Al0.47As forming the cladding, 1500 periods of In0.52Al0.48As forming the multiplication, 110 periods of In0.52A0.48As forming the charging sheet, 50 periods of InAlGaAs forming the grading. The SL2 superlattice layer includes: 50 periods of In0.53Ga0.47As and 1500 periods of 5 mm In0.53Ga0.47As/5 mm GaAs0.51Sb0.40 forming the absorber. The SL3 superlattice layer includes: 45 periods of In0.53Ga0.47As and 500 periods of In0.53Ga0.47As forming the cladding and 20 periods of In0.53Ga0.47As forming a contact.
According to one embodiment, the spectral filter is arranged upstream of the optical system.
According to another embodiment, the spectral filter is arranged downstream of the optical system.
According to an embodiment, the optical system has a first optical path through which a first wavelength light generated by a thermal source is transmitted, and a second optical path through which a second wavelength environmental light reflected from objects is transmitted. The optical system is configured to modify optical characteristics of the light transmitted through the first and second paths such that the light transmitted through the first path do not interfere with light transmitted through the second paths.
According to another general aspect of the invention, a night vision method is provided for imaging an object at or near room temperature and operating in the wavelength region up to 2.5 microns. The method comprises collecting light and focusing the collected light by an optical system onto a photodetector having a cut-off wavelength of 2.5 microns under the room temperature; and selectively filtering out light of wavelength shorter than a predetermined value, thereby gradually shifting imaging from mostly reflection mode to a combined reflection and thermal mode. This feature allows detecting light reflected from and emitted by the object being imaged.
According to one embodiment, the predetermined value of the light wavelength shorter than the wavelength which is filtered out is about 1.8 microns.
According to an embodiment, a cut-off wavelength of an imaging component is selected to modify the ratio between the sensitivity of the imager to light generated by a thermal source and the sensitivity of the imager to light that originates in background light that is being offset from detected objects.
According to an embodiment, the system is configured such that a noise level of the system is low enough so that the system is sensitive to the thermal radiation emitted from the object being imaged within the wavelength range of the photodetector even in the absence of light reflections.
According to an embodiment, the optical detection may be used for detecting firing of firearms.
According to an embodiment, the optical detection may be is used for a night vision system.
According to an embodiment, the absorption layer may include more than 1,000 superlattice layers.
According to an embodiment, the detection array may operate at temperature higher than 270K.
According to an embodiment, the new night vision system includes a spectral filter located in an optical path of light propagating to the detector. The spectral filter may include or be combined with lens for directing light onto the detector. The spectral filter is configured for changing the ratio between the sensitivity of the imager to light generated by a thermal source and the sensitivity of the imager to light that originates in background light that is being offset from detected objects. According to an embodiment, the spectral filter is configured and operable to selectively filter out light of wavelength shorter than about 1.8 microns thereby shifting the system operation from “mostly reflection” mode to “reflection plus thermal” mode. According to an embodiment, wavelengths the correspond to light generated by a thermal source are transmitted along a first optical path in which, whereas wavelengths that correspond to environmental light reflected from objects are transmitted along a second optical path in which. Optics is arranged along at least one of the first and the second optical paths that can modify optical characteristics of the light transmitted in one path and not interfere with the light transmitted along the other path.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows hereinafter may be better understood. Additional details and advantages of the invention will be set forth in the detailed description, and in part will be appreciated from the description, or may be learned by practice of the invention.